The present invention relates to coating particles and fibers, particularly to coating powders and fibers to modify the physical and chemical properties thereof, and more particularly to a process and apparatus for producing adherent coatings on very small particles, powders, and fibers by magnetron sputtering two layers thereon without interface contamination.
Coatings make it possible to modify the physical and chemical properties of powders and small fibers. Coatings provide a method of improving the compatibility of powders, for example, with other materials and in certain processes. Diamond powder, for example, is coated with nickel, for example, to improve its electrical conductivity. Coated diamond is also used in composite grinding tools because it bonds better to most matrix materials than bare diamond. Diamond is one of only a few materials that are available as a coated powder. These coatings are generally prepared by electrochemical plating processes. Although coating uniformity is very good, coating adhesion is only marginal. The known commercial coating processes appear to have problems with flexibility, adhesion, or coverage. Further, there are no known coating processes that are capable of applying uniform and adherent coatings to small powders and fibers in the micron and submicron size. The potential benefit of a coated powder in most applications is determined by the quality of the coating, which includes its adhesive capability. Consequently, the benefits of using coated powders in many material processes has not been evaluated.
The above-referenced coating problems have been overcome by the coating process and apparatus of the present invention. The process of this invention has made it possible to apply an adherent and uniform coating to each particle in a powder sample, for example. Particles with diameters as small as one micron have been successfully coated by this process, which involves a two layer coating without layer interface contamination. The exceptional quality of the coating deposited on small particles and fibers by the process of this invention could lead to the development of improved composite materials and new fabrication processes. The process is carried out using magnetron sputtering of a first thin adhesion layer followed without interruption by at least a second thicker layer of desired material. Prior to coating, the particles and fibers are cleaned by a plasma cleaning operation. Uniformity of coating is maintained by agitation of the particles using the apparatus of the invention.
It is an object of the present invention to provide particles and fibers with a uniform and adherent coating.
A further object of the invention is to provide adherent coatings, on materials that have very little chemical affinity and exhibit poor adhesion to the material of the particle or fiber (e.g. copper to carbon).
A further object of the invention is to provide a process for coating particles in the micron size range with adherent and uniform coatings.
A further object of the invention is to provide a magnetron sputtering process for coating particles and fibers.
A further object of the invention is to provide an xe2x80x9cin-situxe2x80x9d magnetron sputter-coating process capable of applying multiple coatings without interface contamination.
A further object of the invention is to provide a process for coating which involves movement of the material to be coated during the entire coating operation.
A further object of the invention is to provide an apparatus for carrying out the process of the invention.
Another object of the invention is to provide a coating process for small particles and fibers which involves agitation of the material being coated while applying the adherent coatings by in-situ magnetron sputtering, without interface contamination.
Another object of the invention is to provide a process for uniformly applying the adherent coating by vibrating or tumbling the particles or fibers during the coating operation.
Another object of the invention involves applying by magnetron sputtering material an initial coating of a material for adhesion followed by (one or more coatings) of other materials without interruption between the coats.
Other objects and advantages of the invention will become apparent form the following description and accompanying drawings. The invention broadly involves a process and apparatus for coating powder, small particles, and fibers. The process involves the following operational sequence: vibrating or tumbling the powder, particles, or fibers to promote coating uniformity; removing adsorbed gases and static charges from the powder, particles, or fibers by an initial plasma cleaning; and coating the powder, particles, or fibers with one or more layers of metals or ceramics by magnetron sputtering. In the magnetron sputtering operation the powder is first coated with a layer of a reactive metal for adhesion, and additional materials are deposited by in-situ magnetron sputtering to prevent contamination at layer interfaces. This combination of sputtering operations gives the process a unique capability for depositing uniform and adherent coatings on powder, small particles, and fibers. By depositing the additional layer or layers sequentially or simultaneously with the adhesion layer, in-situ and without interruption, the formation of a contaminated interface that could adversely affect the coating adhesion is prevented. Prior to coating, gases adsorbed on the surface of the powder, particles, or fibers and any static charges thereon are removed by a plasma cleaning process which involves sputtering a magnetron source at low power in helium or hydrogen gas at low pressure for a specified time period after which the gas is changed to the sputtering gas (e.g. argon). Coating begins immediately after the plasma cleaning when the gas is changed to argon. In both the cleaning and coating operation, the powder, particles, or fibers are agitated in a tumbler or vibration apparatus to ensure uniformity of cleaning and coating.
A potential use for this invention is improved bonding of powders, particles, and fibers to other materials, such as the matrix material of various composites. A diamond-copper composite alloy with an isotropic thermal conductivity higher than any material except diamond was produced with diamond powder coated with an adhesion layer and a copper alloy. See copending U.S. application Ser. No. 08/247,090 filed May 20, 1994, now U.S. Pat. No. 6,264,882 entitled xe2x80x9cComposite Material Having High Thermal Conductivity And Process For Fabricating Same,xe2x80x9d assigned to the same assignee. Also, metal coatings applied by this process can be used to increase the electrical conductivity of diamond and ceramic powders to improve the bonding of these materials in composite grinding tools or on cutting surfaces of grinding tools for polishing and lapping processes.